1. Field of the Invention
The present invention relates to a projector for projecting images onto a projection surface such as a screen or the like, and more particularly to a projector for correcting distortion of images to be projected onto a projection surface and projecting distortion-corrected images onto the projection surface, and a method of correcting such distortion of images to be projected onto a projection surface.
2. Description of the Related Art
Projectors are often limited to certain installation positions in the environments in which they are used, and project images onto a projection surface such as a screen or the like along an optical axis that is oblique to the projection surface. In such a case, when an image having a square image area is projected onto the projection surface, the projected image suffers a trapezoidal or quadrilateral distortion. Therefore, conventional projectors have a distortion correcting circuit for performing a keystone distortion process to correct a projected image with trapezoidal distortion into a square image.
In recent years, projectors are in general use to have a distortion correcting circuit for automatically correcting image distortion in the vertical direction of the projection surface and manually correcting image distortion in the horizontal direction. One conventional projector of this type is a liquid crystal projector having an acceleration sensor for detecting a vertically inclined angle of the optical axis along which images are projected onto the projection surface, as disclosed in Japanese laid-open patent publication No. 2003-5278, for example.
Other conventional projectors which have found widespread use have, in addition to the distortion correcting circuit, an optical zoom mechanism for enlarging and reducing projected images on the projection surface.
With the conventional projectors which have both the distortion correcting circuit and the optical zoom mechanism, the distortion correcting process is performed, supposing the magnification of the optical zoom mechanism being fixed to an intermediate magnification setting between the telephoto end setting and the wide angle end setting. Therefore, these conventional projectors are disadvantageous in that the distortion correcting circuit is unable to perform the distortion correcting process properly for the other magnification setting.
The reason why the distortion correcting process cannot be performed properly is that distortion of a projected image changes depending on the zoom setting of the optical zoom mechanism. The manner in which distortion of a projected image changes depending on the zoom setting will be described below with reference to FIG. 1.
FIG. 1 shows projector 51 having an angle t of view at the telephoto end setting and an angle w of view at the wide angle end setting. For the sake of brevity, it is assumed in FIG. 1 that the angle w of view at the wide angle end setting is twice the angle t of view at the telephoto end setting (w=2t), and projector 51 has optical axis 55 along which images are projected, optical axis 55 being vertically inclined to a projection surface by an angle t which is equal to the angle t of view at the telephoto end setting.
As shown in FIG. 1, the vertical lengths a, b, c, d of areas of a projected image on the projection surface which are subtended by angles defined by optical axis 55 and the angles t, w of view are related to each other as a<b<c<d. The relationship a<b<c<d Indicates that when the zoom lens, e.g., the projection lens, is moved by the optical zoom mechanism from the telephoto end to the wide angle end, an area of the projected image above the center thereof is enlarged at a ratio greater than an area of the projected image below the center thereof. When the projector projects an image onto the projection surface along the optical axis that is oblique to the projection surface, therefore, the projected image is distorted differently at the telephoto end setting and the wide angle end setting of the optical zoom mechanism.
FIGS. 2A and 2B of the accompanying drawings show images 56, 57, respectively, which are projected onto the projection surface along the optical axis that is oblique to the projection surface in the vertical direction, and also show corrected images 58, 59, respectively, which are obtained by correcting respective distortions of images 56, 57.
The image projected onto the projection surface by the projector suffers a greater distortion when the zoom lens is moved to the wide angle end as shown in FIG. 2B than when the zoom lens is moved to the telephoto end as shown in FIG. 2A. Consequently, the distortion needs to be corrected differently as the zoom setting changes.
It is difficult for the conventional projectors to correct distortion of projected images which changes depending on the zoom setting. If images are projected onto the projection surface along an optical axis that is oblique to the projection surface in both vertical and horizontal directions, then it is more difficult for the conventional projectors to correct distortion of the projected images which changes depending on the zoom setting.